1. Field of the Invention
The present invention is related to methods and related apparatus for depositing films on a substrate by hollow cathode sputtering. More particularly, the present invention relates to methods and apparatus for depositing oxide and other films by hollow cathode sputtering.
2. Background Art
Numerous methods are known for depositing thin films on a substrate. Such methods include, for example, sputtering, vacuum evaporation, chemical vapor deposition, and the like. Typical substrates that are coated with thin films are glass, ceramics, and silicon wafers. Vacuum evaporation is a low pressure deposition technique in which a material is vaporized by heating. Vacuum evaporation is a line of sight deposition technique in which the vaporized material is then radiated out in straight lines from the source. Chemical vapor deposition is a thin film deposition technique in which a reactive gaseous mixture is heated over a substrate. The elevated temperature causes a chemical reaction to occur from which a desired film is formed. Chemical vapor deposition can be undesirable because of contamination of the deposited films.
Sputtering is a low pressure deposition process in which a plasma containing gas ions and electrons is created by the action of an electric field on gas that is introduced into a deposition chamber. The electric field may be formed by either a dc or rf voltage bias. These ions are accelerated towards a target from which material is removed. This removed material is ultimately deposited on a nearby substrate. Reactive sputtering is a further refinement of the sputtering process in which a reactive gas such as nitrogen, oxygen, hydrogen, H2O, H2Se, CH4, C2H6, C2H2, C2H4, B2H6, PH3, CCl4, CF4, organic monomers like HMDSO, pyrrole and the like are introduced into the deposition chamber. These reactive gases are capable of reacting with the removed target material to form a compound film on the substrate. Accordingly, these reactive gases provide one or more atoms that are incorporated into the film. Reactive sputtering is particularly useful for depositing doped and undoped metal oxides, nitrides, carbides, and the like. However, care must be taken in the reactive sputtering process because such reactive gases may form an insulating layer on the conductive target thereby reducing film growth rate.
The effect of insulating layers on the targets in the sputtering process is generally alleviated by the use of RF power to form the plasma. This type of sputtering is referred to as RF sputtering. It is particularly useful for depositing both insulating and oxide films, but deposition rates tend to be low. In the RF sputtering process, a substrate is placed between two electrodes which are driven by an RF power source. Superimposed on this applied RF field is a DC potential. This DC potential advantageously drives the ions toward the target causing some of the target material to be removed. This removed target material may then react with a reactive gas. Again the removed material ultimately coats the substrate.
A number of sputtering refinements makes this technique even more desirable for the deposition of insulating and oxide films. These refinements include unbalanced magnetron sputtering, the utilization of pulsed dc power, and the use of hollow cathodes. The utilization of hollow cathode sputtering in a gas flow mode is a relatively new technique in which an inert gas such as argon is introduced into a channel in a target cathode. While contained within this channel a plasma is formed that removes atoms from the target. These atoms are eventually swept by the gas flow out of the cathode at which point they may then be reacted with a reactive gas. The continuous flow of the inert gas prevents (or tends to prevent) the reactive gas from entering the cathode and thereby prevents (or tends to prevent) an insulating layer from forming on the target. Although the prior art hollow cathode processes may inhibit the formation of an insulating layer on the target, these processes tend to produce films at unacceptably low growth rates.
Accordingly, there exists a need for improved sputtering methods for depositing thin films and in particular insulating or oxide thin films with high growth rates and reduced formation of insulating layers on the targets used in such sputtering processes.